In a continuous ink jet printer, the continuous jet of ink is expelled from an orifice in a print head to form an ink jet. The ink jet is stimulated by a periodic disturbance applied to the print head to cause the ink jet to reliably break up into an evenly spaced series of drops. Generally, one drop per stimulation cycle detaches itself from the ink jet filament. The trajectories of the drops are controlled by inducing a charge in the conductive ink jet filament at the moment of drop separation by means of a charging electrode located near the point of drop separation. In a "binary" type ink jet printer, drops are either charged or not by applying voltage pulses to the charge electrodes. Charged drops are deflected into a catcher from which the ink is recirculated, and uncharged drops proceed to the ink receiving surface such as paper. In a multiple-deflection type ink jet printer, drops are charged to various levels by pulses of various voltage levels on the charge electrode. The variously charged drops are deflected along a corresponding plurality of trajectories depending upon the amount of charge imparted to the drop.
In such continuous ink jet printing apparatus, the term "phase" is used to describe the time relation between the instant of drop separation and the stimulation cycle. The term "phase" is also used to describe the time relation between the printing pulses applied to the charge electrode and the stimulation cycle.
There are many factors that affect the phase of drop separation from the ink jet filament. Among these factors are temperature, pressure, viscosity and surface tension of the ink, strength of the stimulation signal, and the shape and size of the orifice. Since any of these factors may change with time, it is desirable to provide a means for detecting the phase of drop separation, and for adjusting the phase of the printing pulses in response thereto. In continuous ink jet printing systems having a plurality of ink jets, this problem is compounded, since the phase of drop separation may vary slightly from jet to jet. Furthermore, in the event that the difference of phase between the individual ink jets in a multiple jet printer is greater than some predetermined amount, effective phasing for all the jets may not be possible. It is desirable therefore to detect this condition and generate an alarm signal that can be used to shut down the ink jet printer, or initiate an automatic maintenance cycle.
In prior art continuous ink jet printing apparatus, it is known to detect the phase of drop separation by applying a short charging pulse to the drop charging electrode, and phase shifting the charging pulse with respect to the stimulation cycle while measuring the charge induced on the ink drops. The phase at which the maximum charge is induced on the ink drops corresponds to the phase of drop separation. IBM Technical Disclosure Bulletin Vol. 22, No. 7, December 1979, discloses such a phase detection scheme for use with a method of detecting satellite drop formation. U.S. Pat. No. 4,417,256 issued Nov. 22, 1983 to Fillmore et al. describes the phase detection scheme in conjunction with apparatus for adjusting the amplitude of the stimulation signal. In the apparatus described therein, breakoff phase of the weakest driven and strongest driven ink jet filaments are measured at low stimulation power, and the stimulation power is increased until the measured breakoff phase for the two filaments (i.e. the weakest and strongest driven) is equal.
The breakoff phase of each ink jet is measured as a function of the time of flight of an ink drop, sensed by a wire located near the path of the charged drops. The charge induced in the wire by the passing drop is sensed.
When employed with an ink jet printing head of the type having a relatively large number of ink jets, (e.g. 64) the phase detection scheme proposed by Fillmore et al has the disadvantage of taking a relatively long time to execute, since the phase of each jet is measured individually. Furthermore, because of the relatively weak signal produced by the induced charge on the sensing wire as the charged drop passes, the phase detection scheme proposed by Fillmore et al has the disadvantage of having a relatively low signal to noise ratio.
It is an object of the present invention to provide method and apparatus for sensing the phase of ink drop separation that is free from these disadvantages.
In some types of ink jet printing apparatus, the amplitude of the stimulation signal is adjusted as a preset function of the sensed mechanical vibration of the ink jet head. See U.S. patent application Ser. No. 390,105 filed June 21, 1982 now continuation-in-part of Ser. No. 06/77,102 filed Sept. 17, 1985 in the name of Braun where a piezoelectric transducer generates a feedback signal for controlling the amplitude of stimulation. In the manufacture of the ink jet printing head, the optimum stimulation amplitude is determined, and in operation of the ink jet head a closed loop servo controls the stimulation amplitude. In this type of ink jet, the stimulation amplitude is not varied to adjust drop breakoff phase. nevertheless, it is desirable to adjust the phase of the charging signal so that optimal printing conditions are achieved.
It is another object of the present invention therefore, to provide a method and apparatus for detecting the phase of drop separation in a continuous multi-jet type ink jet printer for adjusting the phase of the printing pulses applied to the drop charging electrodes. Furthermore, if the difference in phase between the first drop to separate and the last drop to separate is too great, no single setting of the phase of the print pulse will result in a reliable operation. Therefore, it is a further object of the invention to provide a means for generating an alarm so that further printing can be halted in the event that the phase difference exceeds a predetermined maximum amount.